1. Field of the Invention
The present invention relates to a technique of recording dots on a recording medium with a dot recording head, and more specifically to a dot recording technique using specific recording modes at upper and lower ends of the recording medium to expand a recordable area on the medium.
2. Description of the Related Art
Serial scan-type printers and drum scan-type printers are dot recording devices which record dots with a recording head while carrying out scans both in a main scanning direction and a sub-scanning direction. In these types of printers (especially, ink jet printers), a different recording scheme from that executed in a middle portion of the recording medium is performed in the vicinity of an upper end and a lower end of the recording medium in order to expand the area in which the recording can be carried out. (The area will be hereinafter referred to as the "record execution area" or the "print area".) In this specification, the recording scheme executed in the middle portion of the record execution area is referred to as "middle area recording scheme". The recording process executed in the vicinity of the upper end of the record execution area is referred to as "upper end recording scheme", whereas the recording process executed in the vicinity of the lower end of the record execution area is referred to as "lower end recording scheme".
Prior to the explanation of the upper and lower end recording schemes, the following describes some of the conventional recording schemes adopted for the middle area scheme. As taught by U.S. Pat. No. 4,198,642 and Japanese Patent Laid-Open Gazette No. 53-2040, for example, a technique called the "interlace scheme" has been developed for improving the image (print) quality of printers of this type.
FIG. 34 is a diagram for explaining an example of the interlace scheme. In this specification, the following parameters are used to define printing scheme.
N: Number of nozzles PA1 k: Nozzle pitch [dots] PA1 s: Number of scan repeats PA1 D: Nozzle density [nozzle/inch] PA1 L: Sub-scanning pitch [inch] PA1 w: Dot pitch [inch]
The number of nozzles N is the number of nozzles actually used to form dots. In the example of FIG. 34, N=3. The nozzle pitch k is the interval between the centers of the recording head nozzles expressed in units of the recorded image pitch (dot pitch w). In the example of FIG. 34, k=2. The number of scan repeats s is the number of main scans in which all dot positions on a main scanning line are serviced. In the example of FIG. 34, s=1, i.e., all dot positions on a main scanning line are serviced in a single main scan. When s is 2 or greater, the dots are formed intermittently in the main scanning direction. This will be explained in detail later. The nozzle density D (nozzle/inch) is the number of nozzles per inch in the nozzle array of the recording head. The sub-scanning pitch L (inch) is the distance moved in 1 sub-scan. The dot pitch w (inch) is the pitch of the dots in the recorded image. In general, it holds that w=1/(D.multidot.k), k=1/(D.multidot.w).
The circles containing two-digit numerals in FIG. 34 indicate dot recording positions. As indicated in the legend, the numeral on the left in each circle indicates the nozzle number and the numeral on the right indicates the recording order (the number of the main scan in which it was recorded).
The interlace scheme shown in FIG. 22 is characterized by the configuration of the nozzle array of the recording head and the sub-scanning method. Specifically, in the interlace scheme, the nozzle pitch k indicating the interval between the centers of adjacent nozzles is defined as an integer not smaller than 2, while the number of nozzles N and the nozzle pitch k are selected as integers which are relatively prime. Further, sub-scanning pitch L is set at a constant value given by N/(D.multidot.k).
The interlace scheme makes irregularities in nozzle pitch and ink jetting characteristic to thin out over the recorded image. Because of this, it improves image quality by mitigating the effect of any irregularity that may be present in the nozzle pitch, the jetting characteristic and the like.
The overlap scheme, also known as the multi-scan scheme, taught for example by Japanese Patent Laid-Open Gazette No. 3-207665 and Japanese Patent Publication Gazette No. 4-19030 is another technique used to improve image quality in color ink jet printers.
FIG. 35 is a diagram for explaining an example of the overlap scheme. In the overlap scheme, 8 nozzles are divided into 2 nozzle sets. The first nozzle set is made up of 4 nozzles having even nozzle numbers (left numeral in each circle) and the second nozzle set is made up of 4 nozzles having odd nozzle numbers. In each main scan, the nozzle sets are each intermittently driven to form dots in the main scanning direction once every (s) dots. Since s=2 in the example of FIG. 23, a dot is formed at every second dot position. The timing of the driving of the nozzle sets is controlled so that the each nozzle set forms dots at different positions from the other in the main scanning direction. In other words, as shown in FIG. 35, the recording positions of the nozzles of the first nozzle set (nozzles number 8, 6, 4, 2) and those of the nozzles of the second nozzle set (nozzles number 7, 5, 3, 1) are offset from each other by 1 dot in the main scanning direction. This kind of scanning is conducted multiple times with the nozzle driving times being offset between the nozzle sets during each main scan to form all dots on the main scanning lines.
In the overlap scheme, the dots of each main scanning line are not all recorded by the same nozzle but by multiple nozzles. Even when the nozzle characteristics (pitch, jetting characteristic etc.) are not completely uniform, therefore, enhanced image quality can be obtained because the characteristics of the individual nozzles is prevented from affecting the entire main scanning line.
In the interlace scheme and the overlap scheme discussed above, the sub-scan feed is carried out by the fixed feed amount L corresponding to a plurality of dots.
The upper and lower end recording scheme is a special printing scheme that is executed in the vicinity of the upper end and the lower end of the record execution area, in order to expand the record execution area of the printer as much as possible. The upper and lower end recording scheme may be carried out, for example, according to the technique disclosed in JAPANESE PATENT LAID-OPEN GAZETTE No. 7-242025, the disclosure of which is herein incorporated by reference. FIG. 9 of this reference shows that printing by the interlace scheme is carried out in the middle portion of the record execution area and that printing by "minute feeding" (the sub-scan feed of one dot) is carried out in the vicinity of the lower end of the record execution area. The lower end of the record execution area is expanded as the result of the lower end recording scheme.
The "minute feeding" shifts the nozzle by one dot at a time in the sub-scanning direction. Irrespective of the type of the sub-scan feed adopted for the middle area scheme, the "minute feeding" applied for the lower end recording scheme can expand the lower end of the record execution area to a desired region. There is, on the other hand, a demand for the recording scheme that applies a sub-scan feed other than the "minute feeding" for the lower end recording scheme. For example, in order to improve the picture quality, there is a demand for the recording scheme whose sub-scan feed amount is a plurality of dots, like the interlace scheme and the overlap scheme. When the sub-scan feed other than the "minute feeding" is applied for the lower end recording scheme, some types of the sub-scan feed adopted for the middle area scheme may cause an unprintable area within the record execution area, which is supposed to be expanded by the lower end recording scheme. In the conventional technique, the sub-scan feed other than the "minute feeding" may thus not be applicable for the lower end recording scheme. Another problem is that application of the sub-scan feed other than the "minute feeding" for the lower end recording scheme makes the lower end recording scheme too complicated in some cases.